The present invention relates to methods for producing nitrogen trifluoride which is useful as a dry etching gas or cleaning gas.
Nitrogen trifluoride (NF.sub.3) is a colorless gas under normal conditions and has a boiling point of about -129.degree. C. This compound has conventionally been used as a fluorine source in the preparation of fluoroolefins and also as an oxidizer for a high-energy fuel. Furthermore, much attention has recently been devoted to the usefulness of this compound as a dry etching gas, which is able to leave no residue during the production of semiconductors, and as a cleaning gas for devices of chemical vapor deposition (CVD).
Nitrogen trifluoride is produced usually by direct fluorination of ammonia in vapor phase or by electrolysis of ammonium hydrogenfluoride. Vapor phase reaction between hydrogen azide and oxygen difluoride is also known. U.S. Pat. No. 3,304,248 proposes to carry out reaction between nitrogen and fluorine by forcing nitrogen gas heated to a temperature above 1,000.degree. C. to pass through a plasma arc and simultaneously introducing fluorine gas into a post-arc region very close to the anode. The reactions in these methods are vapor phase reactions which are relatively violent and not easy to control. Furthermore, according to these methods, the yield of nitrogen trifluoride is relatively low.
Japanese Patent Second Provisional Publication JP-B55-8926 (1980) proposes to prepare nitrogen trifluoride by reaction between ammonium hydrogenfluoride in molten state with fluorine gas. This method, however, does not seem industrially favorable firstly because the gas-liquid reaction in this method is not so easy to control and causes significant corrosion of the apparatus and also because the yield of nitrogen trifluoride is relatively low.
U.S. Pat. No. 4,543,242, corresponding to Japanese Patent First Provisional Application JP-A-60-71503 (1985), discloses the production of nitrogen trifluoride with good yields by reaction between fluorine gas and an ammonium complex of a metal fluoride, which is in solid phase. This reaction is easy to control. However, it is further desired to provide a method for producing nitrogen trifluoride with good yields better than those of the above-mentioned methods.